1. Field of the Invention
The present invention relates to a surge absorber used for protecting an electronic component connected to a circuit receiving an abnormally high AC voltage or DC voltage. More particularly, the present invention relates to a surge absorber comprising an integrated gap-type surge absorbing element and a varistor.
2. Description of the Related Art
Gap-type surge absorbing elements are broadly classified into microgap-type discharge tubes and gap-type discharge tubes. A microgap-type discharge tube has a columnar ceramic body having micro gaps formed on the circumferential surface thereof covered with a conductive film. A pair of cap electrodes, with lead wires, cap the hot ends of the ceramic body. An insulating tube contains therein the ceramic body and the cap electrodes. The insulating tube is sealed after being filled with an inert gas. A gap-type discharge tube comprises a gas-filled insulating tube having a pair of electrodes sealing the opposed ends of the tube. The electrodes form a gap that is bridged by plasma when high voltage is applied between the electrodes.
These gap-type surge absorbing elements, having a high insulation resistance, are characterized by a low level of leakage current. Current may continue to flow through the low-impedance plasma path established by the surge voltage between the terminal electrodes after the completion of surge discharge driven by the relatively low source voltage of the circuit being protected. This is called the follow current.
On the other hand, a semiconductor type surge absorbing element using, for example, a zinc oxide varistor, does not rely on plasma conducting for discharging a surge. Thus, such a semiconductor type surge absorbing element does not suffer from follow current. However, a semiconductor type surge absorbing element has the drawback that its leakage current increases at high temperature. To avoid this inconvenience, the zinc oxide varistor may be resin-molded. To take advantage of the properties of both types of devices, semiconductor type surge absorbing elements may be used in combination with a gas discharge surge absorbing element.
Referring to FIG. 3, a prior-art method for producing a combination surge protector includes electrically connecting a lead wire 2 of a gap-type surge absorbing element 1 in series with a lead wire 4 of a varistor 3 using a connecting member 5 such as, for example, a metallic clamp. A case 6, about gap-type surge absorbing element 1 and varistor 3 may conveniently be filled with resin, with lead wires 2 and 4 extending outward therefrom for connection into a circuit (not shown).
The foregoing combination surge absorber requires preassembling gap-type surge absorbing element 1 and varistor 3 in series using connecting member 5 to join their lead wires 2 and 4. Then, the preassembly is placed in case 6 which is then filled with resin. The result is an inconvenient and complicated manufacturing process. A further disadvantage of this method is that gap-type surge absorbing element 1 and varistor 3 cannot be integrally combined into a compact form, and therefore require a relatively large case 6 to contain them.
Alternatively, when a wide enough space is available on a printed circuit board (not shown) gap-type surge absorbing element 1 and varistor 3 may be mounted directly on the print circuit board and connected in series by wiring on the circuit board.
Individual mounting of the gap-type surge absorbing element 1 and varistor 3 on the print circuit board leads to a high packaging cost and poses the problem that its use is limited to applications where a wide packaging space is available.